simply reopen ApplicationController instead of using include/extend. As long as the file is not named "application_controller.rb", then it will load. Rails does not load files from engine /app that have the same name as files in container application /app dir

app/controllers/impressionist_appliation_controller.rb

@@ -0,0 +1,54 @@
+require 'digest/sha2'
+
+class ApplicationController < ActionController::Base
+  before_filter :impressionist_app_filter
+
+  def self.impressionist(opts={})
+    before_filter { |c| c.impressionist_subapp_filter opts[:actions] }
+  end
+    
+  def impressionist(obj,message=nil)
+    unless bypass
+      if obj.respond_to?("impressionable?")
+        obj.impressions.create(message: message,
+                               request_hash: @impressionist_hash,
+                               ip_address: request.remote_ip,
+                               user_id: user_id)
+      else
+        raise "#{obj.class.to_s} is not impressionable!"
+      end
+    end
+  end
+
+  def impressionist_app_filter
+    @impressionist_hash = Digest::SHA2.hexdigest(Time.now.to_f.to_s+rand(10000).to_s)
+  end
+
+  def impressionist_subapp_filter(actions=nil)
+    unless bypass
+      actions.collect!{|a|a.to_s} unless actions.blank?
+      if actions.blank? or actions.include?(action_name)
+        Impression.create(controller_name: controller_name,
+                          action_name: action_name,
+                          user_id: user_id,
+                          request_hash: @request_hash,
+                          request_hash: @impressionist_hash,
+                          ip_address: request.remote_ip,
+                          impressionable_type: controller_name.singularize.camelize,
+                          impressionable_id: params[:id])
+      end
+    end
+  end
+    
+  private
+  def bypass
+    Impressionist::Bots::WILD_CARDS.each do |wild_card|
+      return true if request.user_agent.include? wild_card
+    end
+    Impressionist::Bots::LIST.include? request.user_agent
+  end
+    
+  def user_id
+    @current_user ? @current_user.id : nil
+  end
+end

app/controllers/impressionist_controller.rb

@@ -1,60 +0,0 @@
-require 'digest/sha2'
-
-module ImpressionistController
-  module ClassMethods
-    def impressionist(opts={})
-      before_filter { |c| c.impressionist_subapp_filter opts[:actions] }
-    end
-  end
-    
-  module InstanceMethods
-    def self.included(base)
-      base.before_filter :impressionist_app_filter
-    end
-    
-    def impressionist(obj,message=nil)
-      unless bypass
-        if obj.respond_to?("impressionable?")
-          obj.impressions.create(message: message,
-                                 request_hash: @impressionist_hash,
-                                 ip_address: request.remote_ip,
-                                 user_id: user_id)
-        else
-          raise "#{obj.class.to_s} is not impressionable!"
-        end
-      end
-    end
-
-    def impressionist_app_filter
-      @impressionist_hash = Digest::SHA2.hexdigest(Time.now.to_f.to_s+rand(10000).to_s)
-    end
-
-    def impressionist_subapp_filter(actions=nil)
-      unless bypass
-        actions.collect!{|a|a.to_s} unless actions.blank?
-        if actions.blank? or actions.include?(action_name)
-          Impression.create(controller_name: controller_name,
-                            action_name: action_name,
-                            user_id: user_id,
-                            request_hash: @request_hash,
-                            request_hash: @impressionist_hash,
-                            ip_address: request.remote_ip,
-                            impressionable_type: controller_name.singularize.camelize,
-                            impressionable_id: params[:id])
-        end
-      end
-    end
-    
-    private
-    def bypass
-      Impressionist::Bots::WILD_CARDS.each do |wild_card|
-        return true if request.user_agent.include? wild_card
-      end
-      Impressionist::Bots::LIST.include? request.user_agent
-    end
-    
-    def user_id
-      @current_user ? @current_user.id : nil
-    end
-  end
-end

lib/impressionist/engine.rb

@@ -2,15 +2,7 @@ require "impressionist"
 require "rails"
 
 module Impressionist
-  class Engine < Rails::Engine
-    
-    initializer 'impressionist.controller' do
-      ActiveSupport.on_load(:action_controller) do
-         include ImpressionistController::InstanceMethods
-         extend ImpressionistController::ClassMethods
-      end
-    end
-    
+  class Engine < Rails::Engine    
     initializer 'impressionist.extend_ar' do
       ActiveRecord::Base.extend Impressionist::Impressionable
     end